1. Field of the Invention
The invention relates to Discrete Multi-Tone (DMT) modems, and in particular, tone ordering in DMT modems.
2. Description of the Related Art
Generally, when DMT signals are transmitted over a communication channel they are impaired by various types of noise. DMT signals are coded before the transmission and decoded at a receiver to reduce the error rate of the recovery of the transmitted information. The coding is implemented as a Forward Error Correction (FEC) scheme that adds redundant information to DMT signals to allow the receiver to eliminate unlikely sequences of data. Often, DMT signals transmitted over a communication channel are impaired by cross-tone correlated noise (hereafter, referred to as correlated noise) such as Inter-Symbol Interference (ISI), echo, AM radio, and crosstalk from other devices functioning in the vicinity of the modem and similar narrow band noise sources. The performance of DMT modems is reduced by the correlated noise. This performance loss results in lower data rate or higher bit error rate for DMT modems.
Often, convolutional encoding combined with Viterbi decoding is used as an FEC scheme in various DMT communication systems. This FEC scheme improves the channel capacity and data rate; however, it does not perform well when there is correlated noise in the received signal. The effect of the correlated noise on the Viterbi decoding method can be minimized by reordering tones of the DMT signal. The ADSL standard ITU-T Rec. G.992.3 recommends receiver determined tone-reordering. However, the recommendation does not suggest any particular scheme for tone-reordering. One use for the receiver determined tone-reordering is to spread out the correlated noise within the spectrum of the received signal to get a better coding gain from the Viterbi decoder. For N tones in a received signal, there can be N! (N factorial) possible tone orderings; however, not every tone ordering can provide optimal performance of Viterbi coding. Therefore, a system and method is needed to provide an optimal tone-reordering scheme for DMT modems for an optimal Viterbi coding gain.